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Full text of "The manufacture and properties of iron and steel"

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260                         METALLURGY OF IRON AND STEEL.
was very severe on the machinery; second, when the ingot was rolled into one single plate the segregated interior of the mass constituted a very considerable proportion of the finished piece, and it was generally out of the question to cut this part off, as by so doing a piece would be wasted which would be a very large proportion of the whole and which generally would be unsuited for other purposes on account of its dimensions.
Third, it is.not possible io make every heat of steel just the exact composition and physical qualities desired, and if the steel be cast in ingots of a size suited for the making of certain plates, and if, on account of such variations in chemical or physical quality, they are not suited to the purpose for which they are made, they may be unsuited for any other purpose. On the other hand, when large ingots are cast and bloomed in a large mill and cut up into slabs, it is possible to know before the steel is rolled just what are the chemical and physical qualities of the metal, and the slabs may be made to suit the orders on hand. Moreover, the upper part of the ingot may be put into the less important work, while the bottom portion may be used for fire box plates and for other purposes calling for the best material. Tor these reasons the use of a slabbing mill has come into quite general use.
The Pennsylvania Steel Company was the first works in this country to introduce this practice; the Carnegie Steel Company followed with a much larger mill; The Pennsylvania Steel Company then built one of a large size handling an ingot 36 inches by 48 inches, and the Illinois Steel Company and the Lukens Iron and Steel Company have lately followed the example.
It is difficult to say just what should be the size of the slab for a given plate. Theoretically it would seem immaterial whether a 15-inch ingot is cogged to 8 inches and rolled to one-half inch, or whether it is cogged to 4 inches and rolled to the same thickness. The experiments of Mr. Riley point the same way, but they are far > from being comprehensive. If a slab 4 inches thick is not heated to a full heat the plate may be finished at the same temperature as one of the same gauge rolled from a hotter slab of twice the thickness, but in regular practice the thin slabs are sometimes heated hotter than the thick ones, and consequently will be finished at a higher temperature. If carried too far this produces a coarser structure and an-inferior metal, so that it is best to proportion the thickness of "the slab to the thickness of the plate. The exact relation, is of